In a digital scan converter, radar video signals are quantized by an analog to digital converter. The quantized signals are written into a large digital memory, organized in an X,Y addressing scheme by coordinate conversion logic which converts the polar coordinate system of the radar into the cartesian coordinate system of the memory. The memory is then scanned at 60 Hz to refresh a TV monitor.
The location of a radar pulse return may be defined, in polar coordinates, by R.sub.i .theta..sub.i, where R.sub.i is a measure of range and .theta..sub.i is the azimuth angle of the radar antenna at the time of sampling. The function of the coordinate converter is to compute the corresponding X.sub.i Y.sub.i addresses of each pulse return (sample) and write the sample into the correct location in the TV refresh memory.
In practice, this process is subject to certain problems. One problem is that, near the origin, the azimuthal resolution of the radar is greater than that of the display and returns from adjacent radar pulses at the same range map (convert) into the same X,Y location.
Another problem is that, at long ranges, the radar resolution is less than the display. If not compensated for, this will result in portions of the display memory not being loaded with sampled data, creating "spoking" or pinholes in the display.
Furthermore, this spoking will give rise to cluttering of the display with stale information from many previous radar scans, since tolerances on the antenna pointing angle and arithmetic rounding errors will result in different memory locations being addressed every scan.
At intermediate ranges, when the resolutions of the radar and display are equal, there will be a 1 to 1 mapping of samples defined in the two coordinate systems.
The coordinate conversion requirements are therefore:
(a) For every R.sub.i .theta..sub.i, compute the equivalent X.sub.i Y.sub.i
(b) For samples near the origin, establish which of many samples should be written into X.sub.i Y.sub.i
(c) For samples far from the origin, ensure that each display picture cell (pixel) is addressed and loaded with sensible data.
With regard to item (a) above, the standard equations to convert from polar coordinates to cartesian coordinates are: EQU X.sub.i =R.sub.i sin .theta..sub.i (1) EQU Y.sub.i =R.sub.i cos .theta..sub.i (2)
It can be seen that each transformation requires two multiplications. For a 20 RPM, 5 kHz radar, there are 60/20.times.5.times.10.sup.3 =15,000 radar pulses per scan. For a 1000 line resolution TV raster, with the radar in the center of the display, this results in 75.times.10.sup.5 range samples. The maximum time available for each multiplication is therefore 1/2.times.75.times.10.sup.5 =66 nsec. The hardware required to do this is extremely costly and requires a large amount of physical space.